The Cobb packages have 1 part in common, the access port. Thats the part i would pickup first. its just a little computer that plugs into your OBD2 port (under your dash) and talks to your ECU. If your not mechanically inclined then a stage 1 is perfect for you - nothing to bolt on or remove just plug in the cable and flash the map you are interested in using.
If your on the fence about which stage to choose then I would start with a stage 1 drive a bit with it and monitor the car then move to stage 2 if it suits your fancy. If you have a chance send a sample of your used oil to blackstone for testing just to make sure there aren't any underlying issues that might be exacerbated by an increase in boost. I just did stage 1 on my 13 and I am very pleased with the change. If you dont plan to track your car then i recommend a stage 1 tune.
If you have any additional questions about install/configuration i just did a stage 1+SF intake this weekend so its still fresh in my mind.
13 WRX Hatch - Torqued Performance Stage 1+
Slow is Smooth > Smooth is Fast